The 2022 Pakistan floods, resulting from exceptionally intense monsoon rainfall surges, displaced over 30 million people and caused significant economic damage. This rainfall was approximately four standard deviations above the climatological mean and twice the 2010 flood's rainfall. Simultaneous extreme heatwaves in central China and Europe suggest a potential connection. While previous studies linked La Niña to increased Indian monsoon flow and the 2010 Pakistan floods, the 2022 event's magnitude was unexpected, even with a moderate La Niña. The 2010 and 2022 events shared similarities, including La Niña conditions and a tropical-extratropical interaction involving a European blocking high and intensified summer monsoon flow. The study investigates the physical processes behind the 2022 Pakistan rainfall, focusing on the compounding effects of La Niña-induced easterlies, enhanced Arabian Sea southerlies (with a long-term upward trend), and tropical-extratropical interactions, and explores the linkage to heatwaves in Europe and China.

Prior research highlighted La Niña's role in strengthening the Western North Pacific Subtropical High (WNPSH) and influencing Indian monsoon flow. Studies on the 2010 Pakistan floods emphasized the importance of La Niña-induced changes in large-scale circulation and tropical-extratropical interactions, including a northerly flow linked to a European blocking high and intensified summer monsoon flow in the western Indian Ocean. Existing literature also notes enhanced atmospheric perturbations and rising sea surface temperatures (SSTs) in recent decades, potentially intensifying extreme rainfall and heatwaves. Studies have shown the rising SSTs in the Indian Ocean since 1980, increasing moisture availability and transport, potentially amplifying rainfall in mountainous regions like Pakistan. The concurrence of multiple factors in causing extreme events has also been documented.

The study utilized daily precipitation data from the Climate Prediction Center (CPC) and monthly data from the Global Precipitation Climatology Project (GPCP) and Climatic Research Unit (CRU). ERA5 daily and monthly data (horizontal winds, geopotential height, specific humidity) from the European Centre for Medium-Range Weather Forecasts were used. Data were regridded to 2.5° x 2.5° resolution. Monthly sea surface temperature data from the Extended Reconstructed Sea Surface Temperature (ERSST) and daily outgoing longwave radiation (OLR) data from NOAA were also incorporated. A blocking index was calculated based on the 500-hPa geopotential height gradient, identifying atmospheric blocking events. A heatwave index was defined based on the number of grids exceeding the 95th percentile of daily 2-meter temperature. Statistical tests, including Fisher's Z-transformation, were used to assess correlations. Regression analysis examined relationships between monthly mean time series. Percentile ranks evaluated precipitation intensity and temperature. Idealized heating experiments using the LBM simulated atmospheric responses to heating over Pakistan.

The 2022 Pakistan rainfall showed three intense surges. The extreme rainfall coincided with an intensified southwesterly flow from the Arabian Sea and an easterly anomaly linked to a strong WNPSH. The convergence of these flows created favorable conditions for convection. Concurrent anomalous anticyclonic circulations and extreme warmth were observed over Europe and China. The first and third surges involved northward convection propagation from the Arabian Sea and southward penetration of northerly flow from extratropics. A mid-to-upper-level blocking high over northeastern Europe resembled the 2010 event. The blocking was especially pronounced in August and involved enhanced troughs and southerly winds. The interaction between the northerly and southerly flows created a convection-favorable environment. While both 2010 and 2022 were La Niña years with similar features (easterly winds, enhanced WNPSH, Eurasian wave-like perturbations), differences existed in the Indian Ocean Dipole (IOD) and upper-level anticyclone patterns. The 2022 event showed record rainfall, a strong and westward-extended WNPSH, a strong southerly flow over the Arabian Sea, and vigorous tropical-extratropical interaction. Regression analysis showed a significant correlation between Pakistan rainfall and central China T2m (0.44, P<0.01) and northeastern Europe T2m (0.33, P<0.05). The Rossby wave-like teleconnection pattern is distinct from known patterns (SCA, CGT, BOC). Wave activity flux analysis showed eastward wave propagation in July and southeastward propagation from European blocking to central Asia and China in August. Numerical experiments suggest positive feedback between Pakistan rainfall and the China heatwave. A long-term increasing trend in Arabian Sea moisture flux potentially contributed to the heavier 2022 rainfall.

The study demonstrates that the 2022 Pakistan floods resulted from the compounding effects of multiple factors: La Niña-induced easterlies, enhanced Arabian Sea southerlies (amplified by a long-term trend), and the interaction between tropical monsoon flow and extratropical disturbances. The positive feedback between Pakistan rainfall and the China heatwave highlights a potential mutual enhancement between wet and dry extremes. The distinct Rossby wave-like teleconnection pattern linking Europe, Pakistan, and China differs from previously known patterns, suggesting a potentially emerging pattern that requires further research. The increasing frequency of such events under warming conditions warrants further investigation.

The 2022 Pakistan flood was an intensified manifestation of the 2010 event, caused by compounding factors within an upward trend. The study identified a novel Rossby wave-like teleconnection pattern linking the flood to heatwaves in Europe and China. Future research should focus on the mechanisms driving this pattern and its potential impacts on climate extremes across Eurasia, along with further investigation into the positive feedback mechanisms between the extremes and the long-term trends in Arabian Sea moisture flux.

While the study identifies multiple contributing factors and their interactions, the exact nature of the feedback mechanisms between the China heatwave and Pakistan rainfall remains uncertain and requires further investigation using more sophisticated numerical models. The limited time scale and specific regional focus of this study might restrict the generalizability of the findings to other regions and climate systems.